1. Field of the Invention
The invention concerns an antenna for receiving signals transmitted simultaneously via satellite and by terrestrial means.
It applies in particular to receiving digital audio broadcasting (DAB) radio signals although it is naturally not limited to this application and may be used to receive other types of signal (digital radio broadcast information other than audio programs, radiotelephony, etc) or even to transmit radio signals, by application of the principle of reciprocity.
The broadcasting of high quality sound nevertheless constitutes a particularly critical application in respect of performance and the quality that the user can justifiably expect, especially when receiving signals on board a moving vehicle in an urban environment, and it will be shown that the various features of an antenna in accordance with the invention make it particularly well suited to such use.
2. Description of the Prior Art
To alleviate the presence of lateral obstacles which mask reception, especially in an urban environment, the same program is broadcast simultaneously via satellite and by a plurality of terrestrial broadcasting stations.
The conditions under which signals transmitted by these two means are received are entirely different, both with regard to the radiation pattern required and with regard to the bandwidth and type of polarization.
In the case of signals transmitted by terrestrial broadcasting stations, the radiation pattern needs to have maximum gain (in the direction of the main lobe) for a low elevation angle, in the order of 5.degree. to 20.degree., with a wide bandwidth and using vertical polarization whereas in the case of signals transmitted via satellite the elevation angle must be much greater (typically in the order of 60.degree.) and circular polarization must be used. In either case the radiation pattern must be omnidirectional in azimuth.
An object of the present invention is to propose a composite antenna able to receive both types of signal simultaneously despite their very different receiving conditions, which is of simple and compact construction, in particular to enable it to be mounted on the roof of a vehicle, and which offers excellent radio performance.
The starting point for the invention is a so-called "quarter-wave skirt" type antenna, that is to say an antenna adapted to be mounted above an "artificial ground", comprising a vertical cylindrical tube closed at the upper end and a coaxial feed inside the tube, the radiation pattern of this skirt antenna being essentially omnidirectional with a low elevation angle.
An antenna of this kind is described in U.S. Pat. No. 2,531,476, for example. Because of its low elevation angle radiation pattern, an antenna of this kind, which is in any event designed for terrestrial mobile radio communication using vertical polarization, is unable to receive signals from satellites.